VCSEL devices are considered an attractive alternative to conventional edge-emitting laser diodes due to their small size and their potential of being formed in a substantially circular symmetry. Generally, VCSEL devices show a relatively low threshold current, a high modulation efficiency and, if designed so as to emit a substantially circular beam profile, allow to be coupled into optical fibers in a simple fashion. Additionally, the manufacture of VCSEL devices comes along with a parallel and cost-effective production, testing and packaging process, and also offers the possibility of being packed in one and two-dimensional arrays to comply with a plurality of applications such as data communication, sensing applications, and the like.
In other applications, such as laser pumping, free space communication, illumination systems, or other high-power applications, a laser device not only requires a high total output power in the range of several hundred milliwatts, but also necessitates a high output power per chip area to reduce the required chip area and hence the costs per watt output power. In addition, a high wall-plug efficiency is required to keep thermal losses and the requirements on the packaging side low. Consequently, the output power per chip area or the power density and the wall-plug efficiency represent important parameters that may be decisive for the success of VCSELs in such high power applications. Moreover, the wavelength required for specific high power applications may range from visible wavelength for display applications to infrared wavelength for various sensing or pumping applications. For wavelengths that may be transmitted through a corresponding substrate of a VCSEL element, the requirements with respect to high output power have been met by so-called bottom emitting, flipchip-bonded devices having formed therein an oxide aperture. In this respect, it is to be noted that the terms “bottom” and “top” refer to positions or directions with respect to the substrate on which a VCSEL device is formed. Hence, a bottom emitting VCSEL describes a laser device emitting its output power through the substrate. By means of a heat sink, which is closely located to the laser active area, a very efficient heat removal is provided for the bottom emitting configuration so that relatively high output powers may be generated, wherein, however, this technology is limited to the emission of wavelengths for which the substrate is transparent.
In view of this serious drawback it has been proposed to remove the absorbing material in the substrate, wherein issues concerning the reliability and the requirement for additional fabrication steps may render these approaches less than desirable for mass production of VCSEL elements. For this reason, top emitting VCSEL devices represent an attractive possibility for devices emitting at wavelengths corresponding to the absorption range of the substrate. Generally, the output power of a top emitting VCSEL can be increased by enlarging the active area of the VCSEL. This is usually accomplished by correspondingly increasing an aperture that is formed closely to the active area, wherein the aperture typically provides a current confinement and a restriction of the optical field. Frequently, this aperture is formed by an electrically conductive and transparent material layer, the peripheral area of which is selectively oxidized so as to convert the periphery into a non-conductive oxide material. In other approaches, a conductive and transparent material layer may be modified at the periphery by ion implantation so as to reduce the conductivity and the transmittance of the periphery. Presently, top emitting VCSEL devices having an oxide-based aperture seem to be the most promising approach for demanding applications. For example, VCSEL devices having an output wavelength of 980 nm have been fabricated with an oxide aperture size greater than 90 μm, thereby achieving an output power of more than 100 milliwatts. The increase in the output power, however, is accompanied by a significant decrease of the wall-plug efficiency to about half the value of VCSEL devices having a small oxide aperture. Consequently, VCSEL devices having a large active area may not be considered promising for high power applications when a high efficiency is required.
In view of the problems identified above, there exists a need for a VCSEL device that enables high power output with moderately high efficiency without being restricted to transmittance wavelength range of a substrate.